simple multi_curl based function with ability to set a limit on the max number of concurrent connections

paracurl.php

<?php
/**
 * Process any number of cURL requests in parallel, but limit
 * the number of simultaneous requests to $parallel.
 *
 * @param array $urls          Array with URLs to process
 * @param int   $parallel      Number of concurrent requests
 * @param array $extraOptions  User defined CURLOPTS
 * @return array[]
 */
function paraCurl($urls = [], $parallel = 10, $extraOptions = []) {

    // $extraOptions override the hardcoded ones.
    $options = $extraOptions + [
        CURLOPT_RETURNTRANSFER => 1,
        CURLOPT_FOLLOWLOCATION => 1,
        CURLOPT_CONNECTTIMEOUT => 3,
        CURLOPT_SSL_VERIFYHOST => 0,
        CURLOPT_SSL_VERIFYPEER => 0,
        CURLOPT_HEADER         => 1
    ];

    // The curl_multi handle.
    $mh = curl_multi_init();

    // Array with curl handles.
    $chs = [];

    // Create the individual curl handles and set options.
    foreach ($urls as $key => $url) {
        $chs[$key] = curl_init($url);
        curl_setopt_array($chs[$key], $options);
    }

    $curls = $chs;
    $open = null;

    // Perform the requests requests & dynamically (re)fill available slots
    // up to the specified limit ($parallel) until all urls are processed.
    while (0 < $open || 0 < count($curls)) {
        if ($open < $parallel && 0 < count($curls)) {
            curl_multi_add_handle($mh, array_shift($curls));
        }

        curl_multi_exec($mh, $open);
        usleep(11111);
    }

    // Extract downloaded data from curl handle.
    foreach ($chs as $key => $ch) {
        $res[$key]['info'] = curl_getinfo($ch);
        $response = curl_multi_getcontent($ch);

        // Separate response header & body.
        $res[$key]['head'] = substr($response, 0, $res[$key]['info']['header_size']);
        $res[$key]['body'] = substr($response, $res[$key]['info']['header_size']);

        curl_multi_remove_handle($mh, $ch);
    }

    // Close the curl_multi handle.
    curl_multi_close($mh);

    // Finally return all results.
    return isset($res) ? $res : [];
}